This invention relates to a method of polymerizing 1-olefins and especially ethylene by using a catalyst comprising the reaction product of pinacol and a chromium compound in which the reaction product is formed in situ fixed to the support by heating a mixture of support and reactants at an elevated reaction and activating temperature in a non-oxidizing atmosphere or in which the reaction product is first formed and then fixed to the support by the heating in a non-oxidizing atmosphere.
The use of chromium compounds as catalysts particularly in olefin polymerization is well known in the art, yet the pinacol-chromium reaction product catalysts of this invention are believed to be unique and previously unknown. The types of chromium catalysts known in the prior art are as follows.
One major class of chromium polymerization catalysts is represented by chromium oxide on silica or some other support. As described in U.S. Pat. No. 2,825,721, either chromium oxide or a compound which can be converted to chromium oxide by calcination is suitable for the preparaion of these catalysts. Activation by heating is required, and the atmosphere in contact with the catalyst during this heating preferably contains some oxygen, although it is stated that inert gases like nitrogen can be used. Whatever the activation conditions, a significant property of this type of catalyst is the hexavalent chromium content of the activated catalysts prior to the initial contact with hydrocarbons.
A second type of catalyst containing chromium belongs to the family of Ziegler catalysts. Such catalysts are disclosed in many patents, for example, U.S. Pat. No. 3,257,332, and they have been described in many publications, one of which is the article by J. K. Stille, Chemical Reviews, Vol. 58, pp. 541-579 (1958). A distinguishing feature of the Ziegler group of catalysts is the requirement that there be used along with the transition metal compound a reactive cocatalyst. The cocatalyst is an organometallic compound such as aluminum alkyl compound, a reactive metal, or metal hydride.
The catalysts used in the methods of this invention are clearly distinct from these two major types of chromium catalysts. No reactive organometallic cocatalyst or similar reagent is needed for the catalysts we have discovered. This feature makes the catalysts of this invention different from those of the Ziegler group. Furthermore, in our fixation process evidence indicates that any hexavalent chromium (an essential of U.S. Pat. No. 2,825,721) if initially present is destroyed.